Field
Embodiments of the invention relate to the field of processing systems for audio signals in loudspeakers; and more specifically, to processing systems designed to compensate for an undesired amplitude-frequency characteristic of the loudspeaker system.
Background
The sound quality of loudspeakers is known to be affected by the room they are placed in. At lower frequencies (typically below a few hundred Hz, e.g., below 500 Hz), the proximity of boundaries (walls, large furniture) will cause significant boosts and dips in the frequency-dependent acoustic power radiated into the room.
These effects are strongly dependent on the position of the loudspeaker within the room. A corner placement, for instance, will cause a significant increase in radiated acoustic power at low frequencies, causing the sound to be overly bassy or muddy. The position of the listener's ears with respect to room boundaries will affect the perceived frequency response in a similar manner.
In order to compensate for these effects, and produce a neutral or more balanced frequency response, digital equalization may be used. Many commercially available solutions require measurements at or around the listening positions, requiring the user to move a microphone around the listening environment during setup.
Other solutions make use of microphones built into the loudspeaker system that monitor the radiation in the vicinity of the loudspeaker diaphragm in order to infer a global response, e.g. an estimate of the total acoustic power radiated into the room. Such solutions are described in U.S. Pat. No. 7,092,535 B1 and EP 0772374 B1. A drawback of a global equalization is that a specific, desired, frequency response cannot be achieved at any one location in the room. The advantages, however, may make it a desirable solution for many applications:                1) no microphone has to be moved around by the user;        2) a fixed listening position does not have to be assumed, which will not require a new calibration when the user moves;        3) it is more suitable for a multi-listener setup, a room where listeners move around or where several listening positions exist (such as a sofa and a dining table);        4) it significantly lowers the risk of making the frequency response worse at listening positions that were not measured.        
These global equalization solutions require the estimation of pressure and velocity to estimate the radiation resistance Rrad(f), the real part of the radiation impedance Zrad(f), which may calculated as:Rrad(f)=Re{Zrad(f)}=Re{p(f)/U(f)}where p(f) is the pressure in front of the loudspeaker and U(f) is the volume velocity.
In prior art global equalization solutions, the volume velocity has been estimated from the gradient of pressure in front of the loudspeaker, e.g. by taking a measurement at two distinct positions. Methods relying on pressure gradient require strict tolerances on the microphone matching, or require moving parts if a single microphone is to be employed. They also give little room for design freedom in terms of microphone placement.
Another method used in prior art global equalization solutions is to place an accelerometer on the loudspeaker diaphragm. Because the acceleration signal has to be integrated (to produce a velocity signal), any noise in the measurement will cause an accumulated error.
It would be desirable to provide an easier and more effective way to provide a global equalization for a driver to produce a more balanced frequency response responsive to the environment in which the loudspeaker system is placed.